No, any electrolytic or solid polymer, OS-CON and the like, will always be safe, even when paralleled with a film cap. Tom doesn't quite agree with me on this, but I suspect the bad behaviours he's come across are because of film caps at a distance from each other, i.e. the lead inductance between the caps comes into play.

Always remember that ESR is not a locked in characteristic, it varies all over the place depending upon just about everything, so don't get hung up on precise values ...

Well, a positive combination occurs when:
Case 1: The electrolytic is so high efficiency that it can overcome the film cap. There's most of the Os-Con and solid polymer examples.
Case 2: The value of the film cap isn't large enough to fight with the electrolytic. Like matching woofer to tweeter, the lf of the film cap is chosen to sum with the hf of the electro for level response, neither underwhelm nor overdo. Excessive overlap/overdo makes gross distortion unless the film cap value is small enough to avoid that problem.
Case 3: The film cap has poorer efficiency and could never fight with the electrolytic. Classic example 4.7uF Cerafine with 22nF polyester dip cap.
Case 4: A tiny value electro (Nichicon makes bypass electro and they're super easy to use) or a film cap with a series resistor. These are used for a longer gentler taper and decreased matching problems.

But arbitrary 100nF polypro bypass caps is Russian Roulette for the deaf. If the overlap of the hf from the electro sums greatly with the lf of the film cap at two different timeframes, the resulting gross distortion is both far louder and less useful. It will take much extra effort to sum level and that takes exploring values other than 100nF.

I'm mostly agreeing with Tom. Bypassing electrolytic caps with film caps is possible but very time consuming to do a nice job of it.

It isn't only SMPS that can cause a dulling. Linear unregulated can do it too, especially if the umbilical cable is zero length. A general fix could be run the predrive on regs or. . .

Perhaps Halfler already answered this question so many years ago with an economical sort of "dull-blocker"? Here (attached) I've attempted to apply diode drop to the predrive like Hafler would do. Had this been an LM1875 (with 330u caps on the pins), I would have applied a schottky drop for both v+ and v- to the entire chip, due to lack of separate predrive power connection. However, in the photo below, TDA7294 has the predrive power connection available. TDA7294's output stage is at unity, so the 470u is undecided and could be up to 3,300u or more. Due to vexing pinout corrupting PCB layout, the design below is for use with point-to-point via pinbend (rails horizontally across the chip, v- pins bent all the way to chip face, v+ pins bent half that far, ground in the middle, rails across simply) which is much easier than it looks and takes only minutes. The design is not yet finished. It awaits a bit better filtering, perhaps a different NFB cap arrangement via mixer if needed, exploring the worth of anti-miller-comp since the normal settings are sealed away inside, and other explorations and fine tuning which is not yet done.

Caveat: There's more fake TDA7294 than real, and the fakes don't have working protection or high voltage tolerance or high load tolerance, so if you bought your chip from the market, nearly guaranteeing a fake, use an output cap or other speaker protector and use lower voltage and/or lighter load. Typical tweeter crossover protection is insufficient unless an additional cap is put series to the negative. I'd rather we use Authentic TDA7294 from ST's listed vendors, such as Mouser--Those are real and the protection circuits work.

But, the point is. . . have a look at the power circuit. Did I do it like Hafler?

Why are there smaller bypass caps (3.3nF) upstream of the diodes instead of at the supply?

I've never gotten away without paralleling caps it just seems you can't do it blindly. The net result needs to a low impedance which necessitates parallel capacitances but controlled to not have (sharp) resonances.

The net result needs to a low impedance which necessitates parallel capacitances but controlled to not have (sharp) resonances.

The key phrase to add here is "at high frequencies", audio and above. This is where the killer action happens, the amplifying circuit has to be be able to draw sharp, as in equivalent to high frequency, transients of current with minimal glitching of the voltage rails. Otherwise, everything suffers, especially feedback, because it has to be able to work at high harmonic frequencies to be able to snub out incorrect, distortion harmonics. Resonance peaks means that the power supply will effectively work poorly at a possibly critical, high frequency, by having too high an impedance at that point, and that's what you have to work to avoid ...

Huuhh? Desired impedance is zero, at all frequencies, simple as that. Then no matter how much noise is injected into the voltage rails, or current draw, they're rock steady in value. If you don't like the sound of your amp when functioning in this enviroment then there's something wrong with the amp ...

The key phrase to add here is "at high frequencies", audio and above. This is where the killer action happens, the amplifying circuit has to be be able to draw sharp, as in equivalent to high frequency, transients of current with minimal glitching of the voltage rails. Otherwise, everything suffers, especially feedback, because it has to be able to work at high harmonic frequencies to be able to snub out incorrect, distortion harmonics. Resonance peaks means that the power supply will effectively work poorly at a possibly critical, high frequency, by having too high an impedance at that point, and that's what you have to work to avoid ...

Frank

It's getting just a little confusing, maybe. Resonance is a low-impedance effect. "Resonance peaks" might be easy to misinterpret. In a frequency response plot, resonances are peaks. But in an impedance plot, they would be deep, often-sharp nulls. Resonance occurs when the capacitive reactance and the inductive reactance cancel each other, leaving only the series reistance, which is as low as the impedance can get.

So the best way to think of it might be "resonant dips" and "impedance peaks". But even Henry W. Ott alternates between calling the peaks "impedance spikes", "resonant peaks", "resonant spikes", and "anti-resonance spikes".

Resonance is often a good and helpful effect. For digital decoupling caps, where all of the edges for a particular chip usually have the same rise time (for the current pulses that are demanded by the pin being decoupled), and thus all have the same equivalent dominant frequency f = 1 /(π ∙ trise),
the best you can do is to have the decoupling network be resonant at the needed dominant frequency, since that will make the current response of the caps be able to be bigger and faster, more easily and more quickly, which translates to them being able to supply the needed current with better time and amplitude accuracy, with the smallest-possible disturbance of the power rail voltage.

And where decoupling needs to cover a wider range of frequencies, sometimes people attempt to choose multiple values of parallel caps so that their resonances are spread out over the needed frequency range. Problems are usually part of the result, because the multiple capacitors interact to cause high-impedance peaks in between the resonant dips, where high impedances are not wanted.

The resonance dips are usually "a good thing", in a decoupling or bypassing context. They "swallow" problem frequencies, optimally, if located there. They produce current transients optimally upon demand at frequencies where they are needed, if located there.

The impedance peaks (the "anti-resonance" points produced by interactions among multiple C values in LCL networks) are usually "a bad thing". If they are, say, 25 dB-high peaks, then noise and whatever else is around will be 25 dB higher in level, at those frequencies. And if current is needed at those frequencies, the response is very poor. And if an amplifier's high-frequency power rail feedback has content at an impedance-peak frequency, the amplifier would be likely to oscillate at that frequency.

There is usually not much of a problem if the paralleled capacitor values are within a 2-to-1 ratio, because then the spikes usually fall within the dips, greatly reducing their magnitudes. The severe problems occur mainly when the capacitor values differ by an order of magnitude or more, such as when paralleling small-value film caps and large-value electrolytics.

Adding ANY capacitor, anywhere, is always actually the addition of an LCL network, because there is ALWAYS inductance in the capacitor and in its connections. The resulting LC networks ALWAYS have resonant frequencies. And multiple parallel LCL networks (i.e. any two or more paralleled capacitors of two or more different values) ALWAYS also have antiresonance impedance-peak frequencies, between their resonance-dip frequencies.